The purpose of this recommended practice guide is to demonstrate how to determine the uncertainty associated with mean absorbed energy of specimens tested on a Charpy impact machine. We assume that the Charpy machine has successfully met the requirements for both direct and indirect verification as described in the ASTM E 23, Standard Test Methods for Notched Bar Impact Testing of Metallic Materials. We follow the recommendations and procedures in the Guide to the Expression of Uncertainty in Measurement for computing uncertainty.We assume the reader is somewhat familiar with the Charpy machine verification program administered by the National Institute of Standards and Technology.